Optical fiber Bragg gratings are commonly used as sensors of measurands, such as pressure and temperature. Such sensors typically employ a mechanical arrangement that couples an external pressure to compressive or tensile strain of the optical fiber. Thus, a Bragg wavelength of the grating can be related to the external pressure. In order to remove temperature dependence, a second grating with no strain is typically included as well. This second grating is sensitive to temperature but not to strain. A deconvolution of the two grating wavelengths then results in the measurements of pressure and temperature. Another known way of sensing of temperature, pressure, stress and similar other external perturbations is by the use of a single frequency fiber laser. A single frequency fiber laser can be made of a small gain fiber and a pair of Bragg gratings that act as distributed Bragg reflectors (DBR), or a phase-shifted distributed feedback (DFB) laser inscribed on the gain fiber. Since the external perturbations can cause a change in the laser oscillation frequency through the use of suitable mechanical transducer attached to the laser, by measuring the change in wavelength accurately, one can measure the extent of this external perturbation.
Such sensors have two defects that hamper performance. First, they require high performance bonding and mechanical fixtures so that the fiber may be placed under tension in a reliable fashion. Second, they require optical sources and wavelength sensitive readout modules to obtain the pressure and temperature data.
A second deficiency of the prior art is that wavelength-dependent detectors and/or sources are required to extract the measurand. In cases where high sensitivity is desired, either the detector must have high precision or the source must have narrow linewidth. Such sources and detectors are expensive and difficult to maintain in harsh environments. Other drawbacks include a means for coupling various measurands, such as pressure, to the bending of a fiber, a means for sensing bends in the fiber in a robust manner with one grating and RF detection, and a means to measure very small fiber bends such as those arising from acoustic variations.
Accordingly, new and improved sensing methods and apparatuses that overcome the above limitations of the prior art are required.